1. Field of Invention
The invention generally relates to an apparatus, system and method for measuring tensile and/or compressive stresses experienced by an optical fiber.
2. Description of Related Art
Using an in-fiber Bragg grating to measure temperature and strain is a conventional and widely used technique. Such techniques take advantage of periodicity changes in the grating that are caused by temperature and/or strain changes. For example, an increased temperature or axial tension expands the period between grating etchings and causes a corresponding change in the wavelength(s) reflected by the grating.
A recent example of such a system is described in Ecke et al. (U.S. Pat. No. 6,137,565) which uses a plurality of fiber Bragg gratings to accurately determine the absolute physical state (temperature or stain) in an optical fiber.
As is known in the art, sections of optical fiber may be spliced together using an electric arc or other energy source that fuses the respective fiber ends together. Such a splice is commonly referred to as a xe2x80x9cfusion splice.xe2x80x9d To test the structural integrity of such fusion splices, the fused fibers are routinely subjected to a xe2x80x9cpull testxe2x80x9d in which an axial tension is applied to the fused fibers. Indeed, widely-accepted Bellcore standards require such a pull test (see, for example, Generic Requirements For Fusion Splice Protectors GR-1380-CORE, Issue 1, May 1994). This pull test utilizes conventional tensile testing machines which ramp up the tensile load to a predetermined set point or until failure (optical break or optical loss increase sufficient to declare an effective optical break).
Once a fusion splice is formed, the portion of the fiber that was stripped of its protective layers to form the splice should be protected to prevent environmental degradation of the optical fiber. Such environmental degradation is a widely-studied problem and includes water molecules attacking the chemical bonds in the optical fiber.
Various forms of splice protectors exist including coating the bare fiber with a protective material such as a ultra-violet cured acrylate material. Generally speaking, a splice protector is a mechanical device or fiber coating that provides at least environmental protection to a single fiber or a set of fibers. Splice protectors may also provide mechanical protection to the fusion splice such as inhibiting bending and increasing the tensile strength. As is also known in the art, bending optical fiber affects the useful life of the fiber and exceeding a minimum bend radius can irreparably damage the fiber.
To provide a measure of environmental and mechanical protection, various other forms of splice protectors have been invented. These splice protectors come in various forms such as a hot-melt adhesive tube into which the spliced fibers are threaded. The hot-melt adhesive tube may also be situated inside a heat-shrink tube alone or together with a strength member such as a metal rod. When heat cured, the hot-melt adhesive bonds to the fiber and provides a hermetic seal while the heat-shrink tube shrinks down and encapsulates the fusion splice. In the process, the fusion splice and optical fiber within the splice protector is subjected to a compressive stress.
To date, no method or technique exists for accurately measuring this stress and comparing the stress applied to the fiber inside the splice protector to the stress outside the splice protector.
Other types of splice protectors include clam-shell protectors composed of two hinged plates that may be provided with grooves that accept the fiber and encapsulating protectors which includes a splice holder into which is poured a potting compound. Such clam-shell and encapsulating protectors may also stress the fibers to a degree heretofore unknown.
The invention includes an apparatus for comparing tensile or compressive stresses imposed on different parts of an optical fiber by a package, comprising: a fiber Bragg grating provided in the optical fiber; a free section of said fiber Bragg grating, said free section protruding from the package; shielded section of said fiber Bragg grating, said shielded section being shielded by the package, wherein said free section of said fiber Bragg grating provides a measurement reference with respect to said shielded section of said fiber Bragg grating.
In this apparatus, the shielded section of said fiber Bragg grating may be arranged relative to the package such that said shielded section experiences approximately half of a stress profile within the package.
In this apparatus, the fiber Bragg grating may have a length sufficient to and is axially arranged relative to the package such that said shielded section of said fiber Bragg grating extends approximately halfway into the package.
The invention also includes a system for measuring stress of an optical fiber at least partially disposed in a package, comprising: a fiber Bragg grating provided in the optical fiber; a free section of said fiber Bragg grating protruding from the package; a shielded section said fiber Bragg grating being shielded by the package; a light source optically coupled to said fiber Bragg grating and providing light to said fiber Bragg grating; a measuring device optically coupled to said fiber Bragg grating, said measuring device measuring light received from said free section and said shielded section of said fiber Bragg grating, wherein the measurements made by said measuring device are indicative of stress experienced by the optical fiber.
The measuring device may be an optical spectrum analyzer optically coupled to said fiber Bragg grating, said optical spectrum analyzer receiving light from said free section and said shielded section of said fiber Bragg grating. The system may also include a display device operatively connected to said optical spectrum analyzer, said display device displaying an optical spectrum of the received light.
The inventive system may also include a calculation device operatively connected to said measuring device; said measuring device measuring intensity or phase at a plurality of different wavelengths; said calculation device calculating stress experienced by the optical fiber based on the measurements made by said measuring device.
The calculation device may also calculate a normalized modulus, normalized wavelength shift based on the measurements made by said measuring device.
In an alternative measuring system, a reference grating may be provided on a reference arm of a Michelson interferometer; said fiber Bragg grating being provided on a test arm of the Michelson interferometer. A first fiber tensioner may be provided on the reference arm and used to adjust an optical path length of the reference arm. In this system, the measuring device measures interference fringes caused by interfering wavelengths reflected by said fiber Bragg grating and said reference grating.
The invention further includes a method of analyzing stress of an optical fiber at least partially disposed in a package with a fiber Bragg grating provided in the optical fiber, comprising: arranging the fiber Bragg grating in the package such that the fiber Bragg grating has a free section protruding from the package and a shielded section shielded by the package; supplying light to the free and shielded sections of the fiber Bragg grating; receiving light processed by the free and shielded sections of the fiber Bragg grating; and measuring the received light, wherein the measurements made by said measuring step are indicative of stress experienced by the optical fiber.
The measurements may be displayed or subjected to calculations. For example, the invention may calculate stress experienced by the optical fiber, a normalized modulus, and a normalized wavelength shift based on the measurements made by said measuring step.
The inventive method may also change a condition; and repeat the measuring step after changing the condition. Comparisons of the measurements may then be made before and after the condition is changed.
The types of conditions that may be changed by the invention vary widely. For example, if the package is a splice protector, the condition may be curing of the splice protector. The condition may also be applying axial tension or releasing applied axial tension. Other changeable conditions include time and an environment in which at least at portion of the optical fiber is situated. In this way, measurements may be made over widely varying conditions to fully evaluate the stress applied by a package on the optical fiber at each of the these conditions.
Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.